Up to now, the measurement of the frequency modulation of a laser source was most often achieved using a Michelson or Mach-Zehnder interferometer one of the two arms of which included an acousto-optical modulator. An example of a system of this type is shown in FIG. 1a. It comprises:                a laser source 1, with a controller 11 of a modulation voltage corresponding to a frequency setpoint f0(t), said controller being equipped with a unit 111 for storing digital setpoints and a converter 112 for converting these digital setpoints into analog signals f0(t);        a coupler 12 that samples some of the light emitted in order to send it to an interferometer 2;        a two-arm Mach-Zehnder interferometer 2 with, in one arm, a delay line 21 and, in the other, an acousto-optical modulator (or “AOM”) 22 itself associated with an RF generator 221, and two couplers, one 23 allowing splitting, preferably into two equal portions, and the other 24 allowing light that has passed through the two arms to be recombined;        a photodiode 3 able to convert the light-intensity signal of a beat generated by the interferometer into an analog electrical signal;        a device 4 for measuring the signals delivered by the photodiode 3, which includes a converter 41 for converting these analog signals into digital signals, a converter 42 for converting the analog signals of the generator into digital signals and reciprocally connected to the generator 221, and a unit 43 for storing, at preset times, digital signals generated by the converters 41 and 42;        a unit 5 for processing the stored signals, and transmitting a set voltage to the controller 11; and        a synchronizing device 6 between the storing unit 43, the acousto-optical modulator 22 (via the converter 42 and the generator 221) and the voltage controller 11.        
The frequency is determined by analyzing the signal output from the interferometer; it is a question of a beat signal between the two signals respectively emerging from the two arms.
The signal measured by the photodiode (excluding any DC component) is then:x(t)∝ cos(φ(t)−φ(t−τ)+2πfmaot)where φ(t) is the phase of the laser source, where fmao is the frequency of the acousto-optical modulator and τ is the delay induced by the optical fiber and corresponding to the path difference between the two arms of the Mach-Zehnder interferometer 2. The phase difference φ(t)−φ(t−τ) is characteristic of the frequency f(t) of the laser according to the following relationship:φ(t)−φ(t−τ)=2π∫t τtf(t)dt≅2πτf(t)  (1).
To evaluate the frequency of the laser, it is therefore advisable to calculate:x(t)·exp(−2iπfmaot)then to apply a low-pass filter of cut-off frequency lower than fmao. z(t) is then found such that:z(t)∝ exp(iφ(t)−iφ(t−τ)).
The evaluation of the complex argument of z(t) then finally allows the frequency of the laser to be deduced according to equation (1).
This method relies on the frequency translation induced by the acousto-optical modulator.
Acousto-optical modulators are components that are liable to directly penalize the size, weight, electrical power consumption, reliability and cost of the systems in which they are used. These penalties may also be indirect. For example, it may be necessary to electromagnetically shield the detection chain because of interference caused by the acousto-optical modulator. In addition, it may also be noted that working at high intermediate frequencies requires a more complex detection chain to be used.
Other solutions allow the frequency modulation of the laser source to be measured. The simplest solution is based on the use of an interferometer that is “unambiguous” in the vicinity of the phase quadrature, such as for example a Mach-Zehnder interferometer with a very short delay or an optical resonator of large free spectral range. An example of a system of this type, equipped with a Fabry-Perot resonator is shown in FIG. 1b. It comprises:                a laser source 1, with a controller 11 of a modulation voltage corresponding to a frequency setpoint f0(t), equipped with a unit 111 for storing digital setpoints and a converter 112 for converting these digital setpoints into analog signals f0(t);        a coupler 12 that samples some of the light emitted in order to send it to an interferometer 2;        a Fabry-Perot resonator 2;        a photodiode 3 able to convert the light-intensity signal generated by the resonator 2 into an analog electrical signal;        a device 4 for measuring the signals delivered by the photodiode 3, which includes a converter 41 for converting these analog signals into digital signals, and a unit 43 for storing, at preset times, the digital signals generated by the converter 41;        a unit 5 for processing the stored signals, and for transmitting a set voltage to the controller 11; and        a synchronizing device 6 between the storing unit 43 and the voltage controller 11.        
In this case, the signal output from the interferometer or the resonator and measured by the photodiode may be written:x(t)=A·F(f(t))where A is a proportionality factor depending on the injected power and F a function that is monotonic (and therefore invertible) over the possible range of excursion of the frequency f(t)=fmoy+Δf(t) of the laser. For example, in the case of the short-delay interferometer, if the powers are perfectly balanced, we have:x(t)∝ cos(φ(t)−φ(t−τ))+1≅cos(2πτf(t))+1.
A necessary condition for the function to be invertible is for τ to be sufficiently small that |2πΔf(t)τ|<π.
Thus, this technique is unfortunately not suitable for applications in which a large modulation dynamic range and a high measurement precision are required simultaneously. In addition, the dependency of the proportionality factor A on power may decrease the precision with which the frequency may be measured. Lastly, drift in the system may lead to drift in the measurement (for example loss of the power balance between the two channels of the interferometer or any spectral shift in the response of the resonator).
A last solution consists in simultaneously measuring the phase component and quadrature component of the interferometric signal generated by a two-arm double interferometer. An example of this type of system with a Mach-Zehnder interferometer is shown in FIG. 1c. It comprises:                a laser source 1, with a controller 11 of a modulation voltage corresponding to a frequency setpoint f0(t), which is equipped with a unit 111 for storing digital setpoints and a converter 112 for converting these digital setpoints into analog signals f0(t);        a coupler 12 that samples some of the light emitted in order to send it to an interferometer 2;        a two-arm Mach-Zehnder interferometer 2 with a coupler 23 for splitting, preferably into two equal portions, the light received by the coupler 12, and, in one arm, a delay line 21; in the other arm the light signal is split by a coupler 25 into:                    a phase component that is then recombined using a coupler 241 with light that has passed through the other arm; and            a quadrature component obtained using an element 22, such as a quarter-wave plate, which is then recombined, using a coupler 242, with light that has passed through the other arm;                        a first photodiode 31 able to convert, into a first analog electrical signal, the light-intensity signal of a beat between the delayed signal and the phase component, which are generated by the interferometer;        a second photodiode 32 able to convert, into a second analog electrical signal, the light-intensity signal of a beat between the delayed signal and the quadrature component, which are generated by the interferometer;        a device 4 for measuring the signals delivered by the photodiodes 31, 32, which includes a converter 41 connected to the first diode 31, a converter 42 connected to the second diode 32, and a unit 43 for storing, at preset times, digital signals generated by the converters 41 and 42;        a unit 5 for processing the stored signals, and transmitting a set voltage to the controller 11; and        a synchronizing device 6 between the storing unit 43 and the voltage controller 11.        
In this case, x(t)=A·cos(φ(t)−φ(t−τ))+B and y(t)=C·sin(φ(t)−φ(t−τ))+D are measured, where A, B, C, D are factors dependent on the injected power and the balance of the powers between the channels of the interferometers. Perfect knowledge of these factors allows the following to be measured:
  z  =                                          x            ⁡                          (              t              )                                -          B                A            +              i        ⁢                                            y              ⁡                              (                t                )                                      -            D                    C                      =          exp      ⁡              (                              i            ⁢                                                  ⁢                          φ              ⁡                              (                t                )                                              -                      i            ⁢                                                  ⁢                          φ              ⁡                              (                                  t                  -                  τ                                )                                                    )            
This technique is advantageous because it allows a good compromise between precision and dynamic range to be obtained using interferometers of high finesse (i.e. including a long delay). This technique makes it possible to avoid using any acousto-optical modulators. Nevertheless, it requires a time-invariant quarter wave plate. In addition, it requires the phase to be very precisely controlled, two signals to be acquired simultaneously and good knowledge of the factors A, B, C, D, which depend on incident power and on the balance of the powers of the channels, and which are thus liable to drift over time.